Polymers prepared from fluorinated monomers such as tetrafluoroethylene (TFE) have outstanding physical properties including toughness, chemical and heat resistance, corrosion protection, release, cleanability, low flammability, and weatherability. High molecular weight polytetrafluoroethylene (PTFE) homopolymers provide the highest heat stability among the fluoropolymers but have an extremely low melt flow making fabrication of these polymers difficult. It is known that the melt flow of TFE polymers can be increased by copolymerization with ethylenically unsaturated monomers creating plastics with high enough flow to allow fabrication by melt extrusion or injection molding.
U.S. Pat. No. 2,559,752 to Berry and U.S. Pat. No. 2,952,669 to Bro both disclose polymerization of TFE with ethylenically unsaturated comonomer to make tough polymer which can be hot pressed into transparent film which can be bent 180° without breaking. The polymerization may be carried out in an aqueous dispersion polymerization process using a fluorosurfactant as taught by Berry or in a fluorinated nonaqueous liquid solvent medium as taught by Bro. In U.S. Pat. No. 3,132,123, Harris, Jr. et al. disclose the copolymerization of TFE specifically with perfluoro(alkyl vinyl ether) (PAVE) in a fluorinated solvent to produce melt-processible polymers with sufficient melt flow to permit melt fabrication of useful end products.
In further development of TFE copolymerization, U.S. Pat. No. 3,642,742 to Carlson and U.S. Pat. No. 3,635,926 to Gresham et al. disclose the benefit of using chain transfer agent (CTA) in polymerizing melt processible TFE/PAVE copolymers. Use of a chain transfer agent imparts two highly desirable secondary attributes without decreasing the ease of melt processibility, namely (1) decreased unstable end groups for increased thermal stability and (2) narrowed molecular weight distribution leading to reduced swelling upon extrusion. Carlson conducts the copolymerization in fluorosolvent and Gresham conducts an aqueous dispersion polymerization with fluorosurfactant to produce a colloidally stable dispersion of TFE/PAVE copolymers.
Two general processes for polymerizing tetrafluoroethylene (TFE) to make non-melt processible polytetrafluoroethylene (PTFE) are known—(1) dispersion polymerization and (2) suspension polymerization. Aqueous suspension polymerization has the advantages of using little or no surfactant (dispersing agent), no organic solvents, and the ability to directly isolate the polymer from aqueous medium. All of this is in contrast to dispersion polymerization which uses large amounts of surfactant incurring added expense and requiring recycle and/or disposal. Further, dispersion polymerization requires subsequent steps such as coagulation and washing to achieve isolation of the product.
An effort to produce tetrafluoroethylene/fluoro(alkyl vinyl ether) copolymer by suspension polymerization is described in U.S. Pat. No. 4,499,249 to Nakagawa et al. However, the medium used is a mixed medium of water and a fluorohydrocarbon or a chlorofluorohydrocarbon.
What would be desirable is an improved process for producing tetrafluoroethylene copolymer that eliminates or minimizes the use of surfactants and solvents and permits easy isolation of a copolymer product.